WIMAX OFDMA b iWIMAX - OFDMA basics

1. WIMAX – OFDMA characteristics

2 WIMAX frame structure2. WIMAX frame structure.

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WIMAX OFDMA (I)WIMAX - OFDMA (I)

WIMAX example: Design requirements and tradeoffsWIMAX example: Design requirements and tradeoffs

• Main objective in OFDMA: scalable structure in terms of FFT size and bandwidth maintaining subcarrier spacing fixedbandwidth, maintaining subcarrier spacing fixed.

• To that purpose it is required to choose admissible values for Doppler shift (coherence time) and coherence bandwidth of the channel.shift (coherence time) and coherence bandwidth of the channel.

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WIMAX OFDMA (II)WIMAX - OFDMA (II)

To support mobility 125 km/hr is assumed then maximum Doppler• To support mobility 125 km/hr is assumed, then maximum Doppler shift with 3.5 GHz (in the 2 – 6 GHz range) is

(worst case: 700 Hz, corresponding to 6 GHz).

• ICI power corresponding to is limited (experimentally) to – 27 dB.

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WIMAX OFDMA (III)WIMAX - OFDMA (III)

The coherence time of that channel (a measure of time variation in the• The coherence time of that channel (a measure of time variation in the channel), is the following

• That means an update rate of (approx.)1 KHz is required for channel estimation and equalization.estimation and equalization.

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WIMAX OFDMA (IV)WIMAX - OFDMA (IV)

On the other hand subcarrier spacing design requires a flat fading• On the other hand, subcarrier spacing design requires a flat fading characteristic for worst case delay spread values (20 μs), then the coherence bandwidth is

• This means that for delay spread values of up to 20 μs, multipathThis means that for delay spread values of up to 20 μs, multipath fading can be considered as flat fading over a 10 KHz subcarrier width.

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WIMAX - OFDMA (V)

Without scalability, performance is reduced or cost is increased for low and mid-size channel bandwidthsand mid size channel bandwidths.

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WIMAX - OFDMA frame structure (I)

Like in OFDM, there are three types of OFDMA subcarriers

• Data subcarriers for data transmission,

• Pilot subcarriers for various estimation and synchronizationPilot subcarriers for various estimation and synchronization purposes,

• Null subcarriers for no transmission at all, used or guard bands and DC carriers

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WIMAX - OFDMA frame structure (II)

• A subset of subcarriers (distributed or adjacent), with permuted index, is grouped to form a subchannel.

• Subchannels permutation provide interference averaging benefits and diversity for p p g g yaggressive frequency reuse systems.

• A transmitter is assigned one or more subchannels in DL direction (16 subchannels are supported in UL).

Pilot allocation depends on the subchannelization mode:

Downlink Fully Used Subchannelization (FUSC): pilot tones are allocated first, and remaining subcarriers are divided between subchannels (one set of common pilots).

• Downlink Partially Used Subchannelization (PUSC): all subcarriers (data and pilots) are partitioned in subchannels, then pilots are allocated (each subchannel has their own subset of pilots)

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subset of pilots).

• Uplink subchannelization: same as DL PUSC.

WIMAX - OFDMA frame structure (III)

DL- FUSC: all the data subcarriers are used to createsubcarriers are used to create the various subchannels.

I d f th il t b l i tIndex of the pilots belonging to the variable sets changes from one OFDM symbol to the next, whereas index of pilots , pbelonging to the constant sets remains unchanged.

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WIMAX - OFDMA frame structure (IV)

DL PUSC: all the subcarriers are first divided into six groups.

Permutation of subcarriers to createPermutation of subcarriers to create subchannels is performed independently within each group.

All the subcarriers (except the null) are firstAll the subcarriers (except the null) are first arranged into clusters. In each cluster, the subcarriers are divided into 24 data and 4 pilot subcarriers.

Clusters are then renumbered using aClusters are then renumbered using a pseudorandom numbering scheme. After that, clusters are divided into six groups. A subchannel is created using two clusters from the same group.

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WIMAX - OFDMA frame structure (V)

UL-PUSC: the subcarriers are firstUL PUSC: the subcarriers are first divided into various tiles (tile = 4 subcarriers over 3 OFDM symbols).

The subcarriers within a tile are dividedThe subcarriers within a tile are divided into 8 data subcarriers and 4 pilot subcarriers (tradeoff between higher data rate and more accurate channel t ki )tracking).

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WIMAX - OFDMA frame structure (VI)

UL-AMC: 9 adjacent subcarriers (8 data and 1 pilot) are used to form a bin. 4 adjacent bins constitute a bandconstitute a band.

An AMC subchannel consists of 6 contiguous bins from within the same band.same band.

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WIMAX - OFDMA frame structure (VII)

Slots and frame structure: The MAC layer allocates the time/frequency resources to various users in units of slots which is the smallest quanta of PHYresources to various users in units of slots, which is the smallest quanta of PHY layer resource that can be allocated to a single user in the time/frequency domain.

The size of a slot is dependent on the subcarrier permutation mode.

• FUSC: Each slot is 48 subcarriers by one OFDM symbol.

• Downlink PUSC: Each slot is 24 subcarriers by two OFDM symbols.

• Uplink PUSC: Each slot is 16 subcarriers by three OFDM symbols.

• Band AMC: Each slot is 8 16 or 24 subcarriers by 6 3 or 2 OFDM symbols

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• Band AMC: Each slot is 8, 16, or 24 subcarriers by 6, 3, or 2 OFDM symbols.

WIMAX - OFDMA Frame structure (VIII)

Each frame starts with a preamble f ll d b h F C l H dfollowed by the Frame Control Header (FCH), DL-MAP and UL-MAP (these indicate the current frame structure).

BS periodically broadcasts Downlink Channel Descriptor (DCD) and Uplink Channel Descriptor (UCD) messages to indicate burst profiles (modulation and p (FEC schemes).

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WIMAX - OFDMA Frame structure (IX)

General downlink subframe structure: Downlink Interval Usage Code (DIUC) indicates burst profile

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WIMAX - OFDMA Frame structure (X)

General uplink subframe structure: Uplink Interval Usage Code (UIUC) indicates burst profile

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WIMAX - OFDMA Frame structure (XI)

Ranging in OFDMAR i i li k d th t i t i th lit d li bilit f th• Ranging is an uplink procedure that maintains the quality and reliability of the radio-link communication between the BS and the MS.

• When it receives the ranging transmission from a MS, the BS estimate various radio link parameters (such as CIR SINR and timing and frequencyvarious radio-link parameters (such as CIR, SINR, and timing and frequency offsets) to indicate to the MS any adjustments in the transmit power level or the synchronization parameters.

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WIMAX - OFDMA Frame structure (XII)

Ranging in OFDMA4 t k i iti l ( h i ti ) b d idth t ( ll t )• 4 tasks: initial (synchronization), bandwidth request (allocate resources), handover and periodic ranging.

• CDMA scheme with (BPSK) 144 bit-long ranging codes, that are divided by the BS in 3 sets to identify initial periodic or bandwidth requeststhe BS in 3 sets to identify initial, periodic or bandwidth requests.

• To process an Initial Ranging request, a ranging code is repeated twice and transmitted in 2 consecutive OFDM symbols (optionally 2 codes in 4 consecutive OFDM symbols)consecutive OFDM symbols)

• For Periodic Ranging and Bandwidth Request the options are either 1 or 3 consecutive codes during 1 or 3 OFDM symbols.

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WIMAX - OFDMA Frame structure (XIII)( )

Basic PHY access processp

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WIMAX - OFDMA Frame structure (XIV)

Network entry process

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C l iConclusions

• Scalability is one major design concept in WIMAX – OFDMAScalability is one major design concept in WIMAX OFDMA

• Subcarrier permutation strategies aim to provide frequency selectivity that is not part of basic OFDM concept.y p p

• WIMAX physical layer is characterized by different degrees of freedom in terms of frame size and functions.

• Among the important functions defined by WIMAX PHY there are initial ranging and bandwidth request.

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